The invention relates to a film inking unit for a rotary printing machine and, in particular, to a film inking unit wherein the ink is transported to the printing form from a ductor by a film roller and ink transport rollers. The printing machine can be for example a rotary printing machine for offset printing, water-free offset printing or relief printing.
In film inking units (see e.g. Fachbuch Teschner Offsetdrucktecbnik, p. 403, Fachschriftenverlag, Fellbach, Germany, 8th Edition, 1991), the printing ink supplied by the ductor is taken over by what is called a film roller. As the ductor slowly rotates at an adjustable speed for the purpose of ink dosing, the film roller rotates at the speed of the web. The ductor and the film roller are therefore separated by a distance of approximately 0.05 mm. The ink is subjected to considerable mechanical stress when taken onto the film roller, and heat is produced. As a result, the film roller is heated causing the roller diameter to enlarge. Diameter enlargements of 0.02 mm and more occur. The distance between the film roller and the ductor decreases accordingly, so that the mechanical stress on the ink increases. The reduction in distance makes it more difficult for the ink to pass through the space between the two rollers. Furthermore, the viscosity of the ink becomes thinner and the ink viscosity drops as the temperature increases. Printing ink for water-free offset printing is particularly susceptible to such a viscosity change. Overall, irregularities occur in the ink transfer and are reflected in uneven inking and result in a poor print quality.
U.S. Pat. No. 5,189,960 discloses an inking unit in which a tempered medium flows through at least one roller. The flow of the tempered medium through the roller is performed to maintain a desired temperature of the printing plate on the plate cylinder because excessively high plate temperatures lead, for example, to toning of the non-printed areas.
The object of the invention is to eliminate irregularities in the ink transfer to the film roller and to improve print quality.